The present invention relates to a system for controlling line pressure in an automatic transmission where the pressure is controlled dependent on vehicle speed and throttle opening degree by operating a solenoid operated valve provided in the system.
In a conventional hydraulic circuit for an automatic transmission, the line pressure is controlled by a pressure modifier valve actuated by governor pressure which changes with vehicle speed. As shown in FIG. 5, the line pressure is stepwisely changed from a high level value to a low level value in accordance with the vehicle speed. Namely, in a low vehicle speed range, where engine torque is large, the line pressure is at a high level and in a high vehicle speed range, where the torque becomes smaller, the line pressure is reduced.
Japanese Utility Model Laid Open No. 56-127141 discloses an automatic transmission comprising a hydraulic circuit having three shift valves and an electronic control circuit having a solenoid operated valve to which a signal dependent on vehicle speed and on engine load is applied. The solenoid operated valve is arranged to produce control pressures of three levels. The shift valves are operated by different control pressures respectively, so that the transmission ratio may be changed.
However, in the line pressure control shown in FIG. 5, since the line pressure can only be kept at either of the two levels, there are two regions designated by A and B where line pressures are too high relative to the torque. In such regions, pump loss in an oil pump and shock which occurs at changing of the transmission ratio increase.